Radio won't turn on in the car

TREEN2410

2008 MITSUBISHI COLT

30,000 MILES

The battery went flat in the car we recharged the battery and now the radio wont turn on. When we turn the car on for the first 4 dash came up but we couldn't find the radio code. So we turn the car off and found the radio do went to put the radio in it but the radio won't turn on. How can we get it working